The present invention relates to an engine control system and method and more particularly to a method for adjusting when an engine mode transition in a direct injection stratified charge (DISC) engine control scheme is executed.
In direct injection spark ignition engines, the engine operates with stratified air/fuel operation in which the combustion chamber contains stratified layers of different air/fuel mixtures. The strata closest to the spark plug contain a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures.
The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during the intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to three-way catalytic converters to reduce CO, HC, and NOx. If desired, a second three-way catalyst, known as a NOx trap, is typically coupled downstream of the first three-way catalytic converter to further reduce NOx.
The stratified mode of operation is typically utilized when the engine is operating in light to medium loads. The homogeneous mode of operation is typically used from medium to heavy load operating conditions. In certain conditions, it is necessary to transition from one engine mode of operation to the other. During these mode transitions, it is desired to deliver the requested engine output torque to provide good drive feel. Typically, the determination of when to transition is based on a fuel injection amount, or a desired engine, or powertrain, torque. One such a method, which uses fuel injection amount, is described in U.S. Pat. No. 4,955,339.
The inventors herein have recognized a disadvantage with the above approach. In particular, at higher altitudes, a given engine torque value can be achieved in the stratified mode only by supplying excess fuel with insufficient air. Insufficient air is caused by barometric pressure changes, which provide a lower ambient pressure driving force to fill the engine cylinders with air, i.e., the maximum amount of air that can fill the engine cylinders is reduced as barometric pressure falls, Supplying excess fuel with insufficient air may lead to unacceptable combustion quality with excessive smoke and soot, or may result in emission and driveability degradation. For the transient response during a mode switch, insufficient air may also lead to a torque disturbance since the switch point may not provide equivalent engine output.
The above disadvantages are overcome by a method for controlling an internal combustion engine of a vehicle, the engine operating in at least a first and second operating mode. The method comprises determining a parameter indicative of atmospheric pressure, and selecting one of the first and second operating modes based in part on said parameter.
By adjusting the boundary of the stratified operation when there is less air available at higher altitude and lower barometric pressure, it is possible to obtain improved engine operation. For example, it is possible to obtain improved combustion or smooth transitions between operating modes.
An advantage of the invention is that by having a mode selection that takes into account atmospheric pressure changes, it is possible to obtain improved vehicle performance, since the lower level of engine airflow is considered.
Another advantage of the present invention is that a mode selection that takes into account atmospheric pressure changes, it is possible to operate the engine in acceptable air/fuel ratio ranges and thereby prevent smoke or soot due to degraded combustion.
Other advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.